1. Field Of The Invention
This invention relates to downhole testing apparatus having pumps used for inflating inflatable packers, and more particularly, to a testing apparatus with a progressive cavity inflatable packer pump.
2. Description Of The Prior Art
A known method of testing a well formation is to isolate the formation between a pair of inflatable packers with a flow port therebetween adjacent to the formation. The packers are inflated by means of a pump in the testing string which pumps well annulus fluid or mud into the packers to place them in sealing engagement with the well bore. A variety of such pumps are available.
One type of downhole pump is actuated by the vertical reciprocation of the tubing string connected to the pump. Such a pump is disclosed in Nutter U.S. Pat. No. 3,876,000 and Kisling, III U.S. Pat. No. 3,876,003. This method of reciprocation results in many operational problems, and so other pumps have been developed which are operated by rotation of the tubing string relative to the pump structure connected thereto.
One type of rotationally operated pump uses a plurality of vertically disposed pistons which are driven by a cam structure. Inlet and outlet valves are positioned adjacent to each of the pistons. Typical multiple piston pumps are disclosed in Conover U.S. Pat. No. 3,439,740 and Brandell U.S. Pat. No. 4,246,964, both of which are assigned to the assignee of the present invention. These types of pumps require precise machining and assembly which are relatively expensive and susceptible to damage by impurities in the well fluid. In particular, the valves for each pump can be relatively easily clogged.
A simpler, sleeve-type pump piston is used in the downhole pump of Evans, et al., U.S. Pat. No. 3,926,254, assigned to the assignee of the present invention. This pump uses a plurality of sealing rings of V-shaped cross section for intake and exhaust check valves. In the Evans et al. apparatus, as well as the other pumps described above, the pump piston is in direct contact with well annulus fluid which, because of impurities therein, can result in reduced service life.
In White et al. U.S. Pat. No. 4,706,746, assigned to the assignee of the present invention, a pump is disclosed which uses the more simple sleeve-type pump piston and further includes a diaphragm which separates a piston chamber in which the piston reciprocates from a pumping chamber with inlet and outlet valves therein through which the well fluid is moved to inflate the packers. The piston chamber is filled with clean hydraulic lubricant which promotes longer life for the pump parts. Backup wiper rings are provided to clean the piston of abrasive particulate in the event that the diaphragm is ruptured. Inlet and outlet check valves with resilient annular lips are used, and these are not easily clogged or damaged by abrasives in the well fluid.
The White et al. pump utilizes a pressure limiter which vents around the outlet check valve to the packers at the lower end of the testing string rather than venting to the well annulus.
The same pump is disclosed in White et al. U.S. Pat. No. 4,729,430, also assigned to the assignee of the present invention, which further discloses additional pressure limiter embodiments. Two of these embodiments utilize a pressure limiter piston which reciprocates at a predetermined pressure to increase the volume of the pumping chamber. Another embodiment does not use a specific pressure limiting mechanism, but instead uses a pumping chamber of predetermined volume such that the efficiency of the pump drops to essentially zero when the pressure in the pumping chamber reaches a predetermined level. This necessitates a fairly long tool, and the pressure limiting is a result of this increased volume rather than slippage in the pump itself.
Most of the other pumps of the prior art include relief valves which relieve pressure from the pump to the well annulus. All of these relief devices are relatively complex and add cost to the tool.
In most cases, the prior art pumps have worked well, but are susceptible to clogging and jamming when pumping some fluids such as shales, sand and viscous muds. The pump of the present invention which utilizes a progressive cavity design will handle virtually any fluid that is not corrosive to its components. Progressive cavity pumps are generally known for small pump applications, such as disclosed in Mueller U.S. Pat. No. 4,818,197, assigned to the assignee of the present invention. Progressive cavity pumps have also been adapted for use in downhole tools as production and drill stem testing pumps, such as the Moyno pumps of Robbins & Myers, Inc., and the Norton Christensen NaviPump. These pumps are not used for inflating packers.
Further, the pump of the present invention does not require the expensive and complex necessity of an additional pressure limiting device because the rotor and stator in the progressive cavity pump can be sized such that the pump will not pump fluid once it reaches a specific differential pressure due to internal fluid slippage. That is, the progressive cavity pump itself provides a built-in pressure limitation means. This also allows a more compact tool string and simpler operation.